Controlling output voltage to achieve ultra-low standby power in dim-to-off LED applications

ABSTRACT

An electronic that includes an integrated circuit (IC) configured to regulate an output voltage for powering a light emitting diode (LED). A first transistor is configured to be switched on or off by the IC to inductively couple or decouple a main power supply bus voltage from a primary winding of a transformer to a secondary winding of the transformer connectable to the LED. A second transistor is coupled between the IC and the main power supply bus voltage, and configured to be switched on or off by the IC to selectively provide an IC power supply input voltage to the IC.

BACKGROUND Technical Field

The present application generally relates to regulating powerconverters. More particularly, it relates to controlling output voltageto achieve ultra-low standby power in dim-to-off LED applications.

Related Art

One of the things that smart homes provide is the ability to remotelycontrol home devices over a wireless network such as, Wifi, Z-wave,Zigbee, or Bluetooth Low Energy (BLE). For example, a light-emittingdiode (LED) may be turned-on or turned-off with a switch that is part ofthe smart home wireless network. A remote user device connected to thewireless network may be used to remotely dim the LED over the wirelesslink between the remote user device and the switch.

In a conventional LED switch (e.g., a wall switch), it is not trivial tounderstand that a user simply flips the switch in order to turn the LEDon or off. When the LED is off, power is not needed at the switchbecause the LED is not on. However, in the case of a remotely operatedLED switch, such as that of a smart home, the switch needs to be able torespond to a wake-up signal from the remote user device, for example, toturn on the LED. Thus, the switch needs to be alive (and not shut down)in order to be able to respond to a wake-up signal. Consequently, thisconsumes power, even when the LED is off.

Thus, it is desirable to reduce power consumption by the electronicdevice, especially, when the device is in a standby state. In theexample of the LED switch, when the LED is effectively off, it isdesirable to reduce power consumption to the minimum power that isrequired to keep the switch alive.

As such, there is an ever growing demand to reduce power consumption.For example, in order for a solid-state lighting device, such as an LEDlight, to receive an “Energy Star” certification, the entire device,including the power supply for the LED light and additional auxiliarypower supply for the switch module can consume less than 0.5 W instandby. In other instances, governmental regulations may stipulatepower limits for energy efficiency. For example, California Code ofRegulations Title 20 mandates that the device consumes less than 0.2 Win standby. As such, there is a great push to reduce power consumption,particularly in devices that are in standby modes.

SUMMARY

In dimmable LED lighting applications, there is a growing demand for aso-called “dim-to-off” feature. Dim-to-off allows for a lightingsolution to put the LED into “light-off” state when it receives acommand from a remote user device such as, for example, by a 0-10Vdimmer switch, a microcontroller unit, or BLE wireless module (e.g., aspart of an IOT function for a smart home). During the “light-off” state,the system is in a standby mode, waiting for a wake-up signal from theremote user device to return to the normal lighting mode at any time.Embodiments of the present disclosure explore techniques for keepingpower consumption low in this “light-off” state.

According to a first aspect, an electronic device is described. Thedevice includes an integrated circuit (IC) configured to regulate anoutput voltage for powering a light emitting diode (LED); a firsttransistor configured to be switched on or off by the IC to inductivelycouple or decouple a main power supply bus voltage from a primarywinding of a transformer to a secondary winding of the transformerconnectable to the LED; and a second transistor coupled between the ICand the main power supply bus voltage, and configured to be switched onor off by the IC to selectively provide an IC power supply input voltageto the IC, wherein the second transistor is configured to be switched onin response to: the IC decreasing the output voltage to less than aboutone-third of a nominal operating voltage of the LED, and detecting thatthe IC power supply input voltage is less than a first thresholdvoltage.

According to a second aspect, a method for regulating a voltage isdescribed. The method includes reducing an output voltage coupled with alight emitting diode (LED) to about one-third of a nominal operatingvoltage of the LED with a dimming signal to a voltage regulatingintegrated circuit (IC); and switching on a transistor coupled betweenthe IC and a main power supply bus to apply a main power supply busvoltage to the IC in response to: the IC decreasing the output voltageto less than about one-third of a nominal operating voltage of the LED,and detecting that the power supply input voltage of the IC is less thana first threshold voltage.

According to a third aspect, an electronic device is described. Thedevice includes an integrated circuit (IC) configured to regulate anoutput voltage powering a light emitting diode (LED); and a transistorconfigured to be switched on or off by the IC to inductively couple ordecouple a main power supply bus voltage from a primary winding of atransformer to a secondary winding of the transformer connectable to theLED, wherein the IC is configured to enter a light-off state in responseto receiving a signal comprising a duty ratio less than a predeterminedthreshold, and wherein in response to the IC entering the light-offstate, the transistor is configured to be: switched on when an IC powersupply input voltage is less than a first threshold voltage, andswitched off when the IC power supply input voltage is greater than asecond threshold voltage.

According to a fourth aspect, a method is described. The method includesreducing an output voltage coupled with a light emitting diode (LED) toabout one-third of a nominal operating voltage of the LED with a dimmingsignal, to place a voltage regulating integrated circuit (IC) in a lowpower mode; and switching on a transistor configured to inductivelycouple or decouple a main power supply bus voltage from a primarywinding of a transformer to a secondary winding of the transformerconnectable to the LED, the switching on being in response to: the ICentering the low power mode, and an IC power supply input voltage beingless than a first threshold voltage.

The scope of the invention is defined by the claims, which areincorporated into this section by reference. A more completeunderstanding of embodiments of the invention will be afforded to thoseskilled in the art, as well as a realization of additional advantagesthereof, by a consideration of the following detailed description of oneor more embodiments. Reference will be made to the appended sheets ofdrawings that will first be described briefly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary circuit diagram of a single stage flybackconverter, according to an embodiment of the present disclosure.

FIG. 2 is a graphical representation of the secondary diode current, VCCcharging diode current, and VSENSE voltage when there iscross-regulation in the flyback converter.

FIG. 3 is a graphical representation of the secondary diode current, VCCcharging diode current, and VSENSE voltage without cross-regulation inthe flyback converter.

FIG. 4 is a graphical representation of the output voltage VOUT,transistor Q2, power supply input voltage VCC, and output current IOUT,when the output voltage VOUT is transitioning to a steady state lowvoltage, according to an embodiment of the present disclosure.

FIG. 5 is a circuital diagram of an exemplary converter for regulatingthe output voltage VOUT while avoiding cross-regulation and maintainingthe power supply input voltage VCC above the UVLO voltage, according toan embodiment.

FIG. 6 is a graph of various signals during low power mode.

FIG. 7 is a graph of various signals when approaching light-off state.

FIG. 8 is a graph of various waveforms in light-off state.

FIG. 9 is a graph of various signals during a startup process fromshutdown.

FIG. 10 is an exemplary circuit diagram of a two-stage solution usingflyback and low-voltage buck converter, according to an embodiment.

FIG. 11 is an exemplary circuit diagram of a two-stage solution usingboost and high-voltage buck converter, according to an embodiment.

FIG. 12 is a circuital diagram of an exemplary converter for controllingthe output voltage OUT by regulating the power supply input voltage VCCinto a small band slightly above the UVLO voltage, according to anembodiment.

FIG. 13 is a graph of various signals when regulating the power supplyinput voltage VCC.

Embodiments of the present disclosure and their advantages are bestunderstood by referring to the detailed description that follows. Unlessotherwise noted, like reference numerals denote like elements throughoutthe attached drawings and the written description, and thus,descriptions thereof will not be repeated. In the drawings, the relativesizes of elements, layers, and regions may be exaggerated for clarity.

DETAILED DESCRIPTION

In 0-10V or pulse-width modulated (PWM) dimmable LED lighting solutions,a dim-to-off feature uses an LED driver that is able to turn off the LEDin response to a signal from either a dimmer or a wireless controller.In a light-off state, an integrated circuit (IC) of the converter needsto be alive, in order to respond to the wake-up call at any time toquickly recover the system to the normal lighting mode. It is alsodesired for the system to consume the lowest power when in the light-offstate, since the system may be in the light-off state for an indefiniteperiod. That is, lamps tend to be off for much longer than they are on.Herein the present disclosure, a “light-off” state may also be referredto as a “standby” state. In the light-off state, the LED may notnecessarily be completely off but the output voltage is substantiallyreduced such that the light is not visible, and appears to be off to thehuman eye. For example, when the duty ratio of the PWM signal is lessthan 1%, e.g., about 0.5%, then the LED is in the light-off mode.

Manufacturers generally like to design a single product that iscompatible with many end-products devices to save cost. Similarly,lighting solution manufacturers like to design products that support awide variety of LED driver-end devices. For example, it is desirable toproduce a switch or a converter that can support different LEDdriver-end devices with output voltages that are twice as wide. That is,for example, the same device may be able to support applications havingoutput specifications in the range of 40V 1 A to 20V 1 A. This way, oneplatform design can be used with various modules having different outputvoltages, but the same output current, so as to bring down the overallcost (e.g., design costs, inventory costs, qualification cost, etc.).Given this consideration, if an LED was dimmed from 100% of nominaloperating output to 1%, the LED voltage may drop 70%, by way of example.Thus, for purposes of explaining the various embodiments of the presentdisclosure, approximately “one-third” of the nominal voltage will beused to refer to a voltage that will put, and keep the LED in thelight-off state (i.e., LED off). However, this approximate value ofone-third may be different, as long as it is low enough to keep the LEDoff.

As such it is desirable to achieve a dim-to-off function in a powerconverter by controlling the output voltage to a very low value tomaintain the LED off for various modules, while also achieving ultra-lowstandby power consumption (e.g., 75 mW in standby for a 75 W nominaldesign).

Hereinafter, example embodiments will be described in more detail withreference to the accompanying drawings. The present invention, however,may be embodied in various different forms, and should not be construedas being limited to only the illustrated embodiments herein. Rather,these embodiments are provided as examples so that this disclosure willbe thorough and complete, and will fully convey the aspects and featuresof the present invention to those skilled in the art. Accordingly,processes, elements, and techniques that are not necessary to thosehaving ordinary skill in the art for a complete understanding of theaspects and features of the present invention may not be described.

It will be understood that, although the terms “first,” “second,”“third,” etc., may be used herein to describe various elements,components, regions, layers and/or sections, these elements, components,regions, layers and/or sections should not be limited by these terms.These terms are used to distinguish one element, component, region,layer or section from another element, component, region, layer orsection. Thus, a first element, component, region, layer or sectiondescribed below could be termed a second element, component, region,layer or section, without departing from the spirit and scope of thepresent invention.

Spatially relative terms, such as “beneath,” “below,” “lower,” “under,”“above,” “upper,” and the like, may be used herein for ease ofexplanation to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. It will beunderstood that the spatially relative terms are intended to encompassdifferent orientations of the device in use or in operation, in additionto the orientation depicted in the figures. For example, if the devicein the figures is turned over, elements described as “below” or“beneath” or “under” other elements or features would then be oriented“above” the other elements or features. Thus, the example terms “below”and “under” can encompass both an orientation of above and below. Thedevice may be otherwise oriented (e.g., rotated 90 degrees or at otherorientations) and the spatially relative descriptors used herein shouldbe interpreted accordingly.

It will be understood that when an element or layer is referred to asbeing “on,” “connected to,” or “coupled to” another element or layer, itcan be directly on, connected to, or coupled to the other element orlayer, or one or more intervening elements or layers may be present. Inaddition, it will also be understood that when an element or layer isreferred to as being “between” two elements or layers, it can be theonly element or layer between the two elements or layers, or one or moreintervening elements or layers may also be present.

An exemplary circuit of a single stage flyback converter 100 isillustrated in FIG. 1. The circuit includes a bridge rectifier 101 forrectifying an input AC signal 102 and generating a direct current (DC)voltage from the bridge rectifier 101 on main power supply bus VBUS. Thebridge rectifier is coupled to the flyback converter 100, which iscoupled to an output circuit 103 via a transformer having a primarywinding 104 and a secondary winding 105. The output circuit 103 mayinclude components such as a secondary diode 106, an output capacitor112, and dummy load 107, all coupled to a load (e.g., a light emittingdiode (LED) 108). The primary winding is part of the flyback converter100 and it has a first electrode coupled to a main power supply busVBUS, and a second electrode coupled to a switch (e.g., transistor Q1),which is configured to be selectively turned on and turned off.

When transistor Q1 is turned on, energy is built up in the primarywinding 104, and when transistor Q1 is turned off, the energy from theprimary winding 104 is transferred to the secondary winding 105. Thus,when transistor Q1 is turned on and then turned off, the voltage fromthe main power supply bus VBUS is transferred from the primary winding104 to the secondary winding 105, and ultimately provides an outputcurrent to the LED 108. When the LED 108 receives the output current,the LED 108 turns on to a brightness corresponding to the magnitude ofthe output current.

In some embodiments, a gate electrode of transistor Q1 is coupled to anintegrated circuit (IC) 109 as illustrated in FIG. 1. In this manner,the IC 109 can be configured to apply a voltage corresponding to a highor a low signal to the gate of switch Q1 in order to turn on or turn offswitch Q1, respectively.

In some embodiments, the flyback converter 100 illustrated in FIG. 1includes a transistor Q2 coupled between the main power supply bus VBUSand the IC 109. More particularly, a first electrode of transistor Q2 iscoupled to the main power supply bus VBUS, a second electrode oftransistor Q2 is coupled to a power supply input (VCC) of the IC 109 anda power supply capacitor 113, and a gate electrode of transistor Q2 iscoupled to VIN of the IC 109.

In some embodiments, a pulse width modulated (PWM) signal is provided toinput DIM of the IC 109 to modulate the LED 108 output current based onthe duty ratio of the PWM signal. For example, the LED 108 may be dimmedfrom a full on at 100% nominal current to a very dim LED at 1% of thenominal current by affecting the duty ratio of the PWM signal at inputDIM.

Some of the obstacles faced when precisely regulating the output voltageat low voltages, relative to its nominal voltages (e.g., one-third ofits nominal voltage), include preventing cross-regulation between theoutput voltage VOUT and the power supply input voltage VCC, and keepingthe LED 108 alive by preventing the power supply input voltage VCC (andconsequently output voltage VOUT) from dropping below anundervoltage-lockout (UVLO) voltage below which, the IC 109 would shutdown. Herein the present disclosure, regulating of the output voltage isdescribed, but what is actually regulated is the current. Consequently,the output voltage is regulated because the current is regulated.

As the output voltage VOUT is reduced by lowering the duty ratio of thePWM signal, the output power is significantly reduced due to the V_(out)²/R_(L) relationship, where R_(L) is the output dummy load 107. At thispoint, the primary side of the flyback converter becomes vulnerable tocross-regulation between the output voltage VOUT and power supply inputvoltage VCC.

Cross-regulation is related to the difference between the powerconsumption by the output load (e.g., LED 108) and the VCC load (e.g.,IC 109). That is, assuming that the secondary winding 105 and theauxiliary winding 110 are the same in various aspects (i.e., turns,coupling with primary, etc.), then the IC 109 (via VSENSE) will regulatethe output voltage VOUT when the output voltage VOUT loading is heavierthan the power supply input voltage VCC loading. For example, VSENSE ofthe IC 109 can sense the output voltage VOUT or power supply inputvoltage VCC, depending on which of the current of secondary winding 105or the current of the auxiliary winding 110 dies later.

One technique to avoid cross-regulation when going from normal tolight-off mode is to increase the dummy load 107 by reducing preload toincrease the loading on the output voltage VOUT. However, reducingpreload results in higher resistive losses, which may increase powerconsumption, thus adversely affecting the standby power consumptionrequirements. Cross-regulation may also be avoided by lowering the VCCload (e.g., IC 109) by reducing the power consumed by the IC 109. Byreducing the IC 109 power consumption (e.g., 3 mA to 0.5 mA), the IC 109operates at reduced power in a “low power mode,” and lowers the VCCloading (thus VOUT loading is relatively greater). Consequently, the lowpower mode of the IC 109 contributes to the overall objective ofreducing power consumption by the converter and the LED 108,particularly, when the LED 108 is in the light-off mode. That is, theLED 108 consumes less power because it is in the light-off mode, and theIC 109 consumes less power because it is in the low power mode, allwhile avoiding cross-regulation. Low power mode provides sufficientpower for the IC 109 because the LED 108 is effectively off, and the IC109 needs just enough power to stay alive.

In some embodiments, the power supply input voltage VCC can betemporarily charged to avoid dropping below UVLO, which causes thesystem to shut down. Transistor Q2 may be turned on to charge powersupply input voltage VCC from the main power supply bus VBUS. Forexample, during a transient state when the output voltage VOUT is beinglowered from nominal voltage to a reduced operating voltage (e.g., 100%to 33.3%), transistor Q2 may be turned on by applying a high voltage tothe gate of transistor Q2 from VIN of the IC 109. When Q2 is turned on,current from the main power supply bus VBUS flows through transistor Q2to charge power supply input voltage VCC. In this way, power supplyinput voltage VCC can be charged even while the output voltage VOUT isin a transient state because the charging takes place on the primaryside, without affecting the secondary side where the output voltage VOUTis being discharged. It should be noted, however, that charging from themain bus is not energy-efficient, and therefore should not be used forextended time, such as, for example, after the output voltage VOUTreaches steady state.

If the power supply input voltage VCC is reduced below anundervoltage-lockout (UVLO) voltage of the LED 108, the converter 100(i.e., LED 108 and IC 109) would actually shut down, instead of being ina standby state. Once it shuts down, a wake-up signal would not be ableto restore the IC 109 to turn the LED 108 back on. Thus, it is desirablefor the IC 109 to be in a standby state instead of being shut down sothat the LED 108 can be turned back on through electronic remote means(e.g., BLE, microcontrollers, etc.). Thus, it is important to keep thepower supply input voltage VCC above the UVLO voltage.

By charging the power supply input voltage VCC through transistor Q2from the main power supply bus VBUS, the power supply input voltage VCCcan be maintained above the UVLO voltage threshold to avoid shuttingdown the LED 108. As known by those skilled in the art, the UVLO voltageis a built-in parameter of the IC 109, and may vary among different ICs.

FIG. 2 shows a graphical representation of the secondary diode current,VCC charging diode current, and VSENSE voltage when there iscross-regulation in the flyback converter. As shown, the secondary diodecurrent dies before the VCC charging diode current dies at the kneepoint, shown with line 200. For example, if the load in the secondaryside is lighter, then the output voltage VOUT will be pushed higher dueto energy imbalances. Consequently, the secondary diode current diesfaster due to a relationship shown by Ldi/dt=VOUT for the same secondarypeak current as the output voltage VOUT increases. On the other hand,the IC 109 power consumption can be reduced by reducing the VCC chargingdiode current so that the VCC charging diode current dies before thesecondary diode current at the knee point, as illustrated in FIG. 3 toavoid cross-regulation.

FIG. 4 shows a graphical representation of the output voltage VOUT,transistor Q2, power supply input voltage VCC, and output current IOUT,when the output voltage VOUT is lowered and the transistor Q2 is turnedon to charge the power supply input voltage VCC, when the LED 108 istaken from a normal operating mode (e.g., VOUT=48V) to the light-offstate (e.g., VOUT=16V or one-third of 48V), according to an embodimentof the present disclosure.

With the output capacitor 112 in place on the secondary side, theprocess of lowering (i.e., by discharging) the output voltage VOUT maybe relatively slow, depending on the size of the output capacitor 112.In order to avoid prolonging the discharge process, the frequency of thePWM signal to the DIM input of IC 109 is kept low, e.g., about 25 kHz,thus reducing the energy transfer between the primary winding 104 to thesecondary winding 105 to nearly zero. As a result, power supply inputvoltage VCC may drop faster and go below the UVLO voltage before theoutput voltage VOUT reaches its steady state voltage.

To prevent the power supply input voltage VCC from falling below theUVLO voltage, a first threshold voltage and a second threshold voltagefor the power supply input voltage VCC are set so that the transistor Q2can be turned on and turned off to charge power supply input voltage VCCfrom the main power supply bus VBUS, to keep the power supply inputvoltage VCC between the first threshold voltage and the second thresholdvoltage, according to an embodiment of the present disclosure. In theillustrated embodiment, the first threshold is above the UVLO voltage(e.g., 10.0V) and the second threshold voltage (e.g., 11.0V) is slightlygreater than the first threshold voltage. In this manner, the powersupply input voltage VCC may be maintained above UVLO to preventshutdown during the transient from normal operation state to light-offstate.

Thus, as shown in FIG. 4, as output voltage VOUT and power supply inputvoltage VCC begin to lower, transistor Q2 cycles on and off to keep thepower supply input voltage VCC between the first threshold voltage andthe second threshold voltage. When the output voltage VOUT ultimatelyreaches steady state, the transistor Q2 turns off and remains off sothat power is not consumed through the energy-inefficient main powersupply voltage VBUS. As such, charging through the main power supply busvoltage VBUS through the transistor Q2 is used when entering thelight-off mode from normal mode, but once the system reaches steadystate in light-off mode, the power supply bus voltage VBUS is no longerused. Furthermore, once the IC 109 has entered the low power mode, thePWM signal may be changed to a pulse-frequency modulation (PFM) signalto reduce the energy that is delivered.

FIG. 5 is a circuital diagram of an exemplary converter for regulatingthe output voltage VOUT while avoiding cross-regulation and maintainingthe power supply input voltage VCC above the UVLO voltage, according toan embodiment.

In some embodiments, a PWM Detect Logic 501 measures the PWM inputsignal at DIM input of the IC 109. When the duty-ratio is less than apredetermined threshold value (e.g., 0.5%), which is less than thelowest dimming level of the LED 108 (e.g., 1.0%), power controller 502puts the IC 109 into light-off state. Once the IC 109 is in thelight-off state, the power controller 502 enables low power mode toreduce VCC consumption by the IC 109. The low power mode may be achievedby, disabling unnecessary analog function blocks e.g., cut VDAC andIDAC, VSENSE comparators and ISENSE comparators, slowing down the driverand some of comparators, slowing down digital clock speed (e.g., by 1/16of the original speed), and disabling unnecessary logic (e.g., nothermal detection is required during light-off state).

In some embodiments, a timing logic is used to implement the low powermode and to wake up from the light-off state back to the normaloperating state. FIG. 6 shows an example sequence of signals. The clockfrequency is masked by the low power mode enabling signal LP_EN. WhenLP_EN is high, the low power mode is enabled, and vice versa. In thelight-off mode, the switching frequency is substantially reduced (e.g.,1 kHz). During most of time in the 1 ms switching period, the LP_EN ishigh, and the clock speed is reduced by 1/16 or more (e.g., 1/64) tosave power supply input current ICC, while not jeopardizing the powercontroller 502 due to the hibernation nature of the state. However, whenthe power switch is turned on and turned off, the power controller 502returns to the active state so that it can measure voltage and currentagain. In such case, timing is to ensure IC 109 comes out of the lowpower mode before the switching pulse is turned on, and goes back to thelow power mode after the switching pulse is turned off and allmeasurements are done.

In some embodiments, the first and second threshold voltages aregenerated from a bandgap circuit 503. Referring back to the exemplarycircuit in FIG. 5, the UVLO is 7.5V and the two threshold voltages are10V and 11V, respectively, which are slightly greater than the UVLO.Accordingly, a first comparator 504 is configured to compare the powersupply input voltage VCC with the first threshold voltage level, 10V.Similarly, a second comparator 505 is configured to compare the powersupply input voltage VCC with the second threshold voltage level, 11V.In some embodiments, the outputs from comparator 504 and 505 arecombined and processed by a light-off state control logic 507 to keeppower supply input voltage VCC between 10V and 11V by turning on andturning off transistor Q2 accordingly, to avoid cross-regulation anddropping below the UVLO voltage during the transient stage when theoutput voltage is transitioning from normal voltage to light-off. Afterthe sensed output voltage VOUT at VSENSE is equal to the predeterminedvoltage, the power controller 502 switches transistor Q1 on and off toregulate the output voltage VOUT to the predetermined voltage. Aspreviously provided, once output voltage VOUT achieves steady state,transistor Q2 is cut off and not turned on again to avoid consumingenergy from the main power supply bus VBUS. Comparator 506 is configuredto compare the power supply input voltage VCC with the UVLO voltage,which is 7.5V in this example. By keeping the output of comparator 706positive, power supply input voltage VCC can be kept above the UVLOvoltage.

As provided, output voltage VOUT regulation can be accomplishedaccording to various techniques. A simple way to avoid loop compensationis to use a fixed current command (e.g., 0.3V) for ISENSE input todetermine the pulse width. Then, the VSENSE voltage may be used todetermine switching period. For example, if the sensed voltage isgreater than the target value, the switching period for next cycle willbe increased by a predetermined value (e.g., 6.25%), or decreased by apredetermined value if the sensed voltage is less than the target value.A small hysteresis may be reserved for the reference value to avoidoscillation. FIG. 7 shows the concept on how to control the switchingperiod, and FIG. 8 shows the measured waveforms in the light-off state.As shown, VIN is pulled low to cut off transistor Q2 to save power inthe steady state, while IC 109 is regulating output voltage VOUT at lowlevel to sustain VCC while achieving very low energy consumption.

In some embodiments, when it is desired to turn the LED 108 back on froma shut down state, the power converter 100 is first brought back to thelight-off mode, and then brought to the normal operating mode. FIG. 9illustrates the startup process from shut down to light-off to normaloperation.

FIG. 10 is an exemplary circuit diagram of two-stage solutionincorporating a low-voltage buck converter as a second stage to removedoubled line-frequency ripple in LED current, and FIG. 11 is anotherexemplary circuit diagram of a two-stage solution incorporatinghigh-voltage buck converter, according to various embodiments of thepresent disclosure. In some embodiments, the power supply input voltageVCC may be regulated in the light-off mode while conserving power bymaintaining the power supply input voltage VCC to a voltage slightlyhigher than the UVLO. By regulating the power supply input voltage VCC,output voltage VOUT is also controlled due to the magnetic coupling bythe transformer. Although the output voltage VOUT and power supply inputvoltage VCC are not in a direct linear relationship, they aresubstantially proportional. Thus, a higher output voltage VOUT willresult in a higher power supply input voltage VCC, and a lower outputvoltage VOUT will result in a lower power supply input voltage VCC.Therefore, by controlling power supply input voltage VCC to be a smallvalue just above the UVLO voltage, the resulting output voltage VOUT isalso at its minimum.

Differently from the technique described above with reference to thecircuit in FIG. 1, this technique does not draw power from the mainpower supply bus VBUS. However, this technique relies on transistor Q1,which is the main switch that turns on the power converter, to chargethe power supply input voltage VCC. Thus, transistor Q1 in FIGS. 10 and11 are configured to be selectively turned on and off to turn on or offLED 108 at output voltage VOUT,

FIG. 12 is a circuital diagram of an exemplary converter for regulatingthe output voltage OUT while avoiding cross-regulation and maintainingthe power supply input voltage VCC above the UVLO voltage, according toanother embodiment.

Similarly to the circuital implementation in FIG. 5, a PWM Detect Logic701 measures the PWM input signal at DIM input of the IC 109. When theduty-ratio is less than a predetermined threshold value (e.g., 0.5%),which is less than the lowest dimming level of the LED 108 (e.g., 1.0%),power controller 502 puts the IC 109 into light-off state. Once the IC109 is in the light-off state, the power controller 502 enables the lowpower mode to reduce VCC consumption by the IC 109. The low power modemay be achieved by, disabling unnecessary analog function blocks e.g.,cut VDAC and IDAC, VSENSE comparators and ISENSE comparators, slowingdown the driver and some of comparators, slowing down digital clockspeed (e.g., by 1/16 of the original speed), and disable unnecessarylogic (e.g., no thermal detection is required during light-off state).

In some embodiments, a critical timing logic is used to implement thelow power mode and to wake up from the light-off state back to thenormal operation state. FIG. 6 shows an example sequence of signals. Theclock frequency is masked by the low power mode enabling signal LP_EN.When LP_EN is high, the low power mode is enabled, and vice versa. Inthe light-off mode, the switching frequency is substantially reduced(e.g., 1 kHz). During most of time in the 1 ms switching period, theLP_EN is high, and the clock speed is reduced by 1/16 or more (e.g.,1/64) to save power supply input current ICC, while not jeopardizing thepower controller 702 due to the hibernation nature of the state.However, when the power switch is turned on and turned off, the powercontroller 702 returns to the active state so that it can measurevoltage and current again. In such case, timing is critical to ensure IC109 comes out of the low power mode before the switching pulse is turnedon, and goes back to the low power mode after the switching pulse isturned off and all measurements are done.

In some embodiments, the first and second threshold voltages aregenerated from bandgap 703. In the illustrated example, the UVLO is 7.5Vand the two threshold voltages are 8.5V and 9.5V, respectively, whichare slightly greater than the UVLO. Note, that the first and secondthreshold voltages according to this technique are closer to the UVLOvoltage than the thresholds in the implementation of FIG. 5. In theimplementation of FIG. 5, the first and threshold voltages do notnecessarily have to be as close to the UVLO voltage (with respect to theimplementation of FIG. 12) because at steady state, output voltage VOUTis regulated by VSENSE. Accordingly, a first comparator 704 isconfigured to compare the power supply input voltage VCC with the firstthreshold voltage level, 8.5V. Similarly, a second comparator 705 isconfigured to compare the power supply input voltage VCC with the secondthreshold voltage level, 9.5V. When the output of comparator 704 isnegative, the IC 109 may begin to switch (i.e., turn on and off) at apredetermined frequency (e.g., 25 kHz) at fixed peak current commanduntil power supply input voltage VCC increases and the output ofcomparator 705 becomes positive when transistor Q1 is turned off. Byrepeating this process, power supply input voltage VCC may be maintainedbetween 8.5V and 9.5V. In this manner, the IC 109 power consumption isreduced as well as the overall power consumption, thus the outputvoltage VOUT can be controlled to a small voltage without relying on asubstantial dummy load to achieve the low output voltages to turn offthe LED 108 and keep them in the light-off mode. Comparator 706 isconfigured to compare the power supply input voltage VCC with the UVLOvoltage, which is 7.5V in this example. By keeping the output ofcomparator 706 positive, power supply input voltage VCC can be keptabove the UVLO voltage.

Accordingly, at steady-state, the IC 109 operates at low power levelwith low current consumption, which does not consume much power (e.g.,given VCC=10V and ICC=0.5 mA, the IC 109 power consumption is only about5 mW). With low VCC power consumption, cross-regulation can be easilyavoided without adding a significant amount of dummy load at the output.As a result, the total power consumption may be reduced to about 75 mWor less in standby mode. power can be achieved even at high-line inputvoltage.

FIG. 13 is a graphical representation of regulating power supply inputvoltage VCC and keeping the LED 108 off. As shown, the IC 109 regulatespower supply input voltage VCC to a voltage band (e.g., 8.5V to 9.5V)that is slightly higher than the UVLO voltage (e.g., 7.5V). For everyobserve period To (e.g., 30 us), system checks the output of comparators704 and 705 as illustrated with signal ISENSE. Power supply inputvoltage VCC is kept above UVLO voltage of 7.5V, while maintaining itbetween 8.5V and 9.5V.

The terminology used herein is for the purpose of describing particularembodiments and is not intended to be limiting of the present invention.As used herein, the singular forms “a” and “an” are intended to includethe plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes,” and “including,” when used in thisspecification, specify the presence of the stated features, integers,steps, operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof. As used herein,the term “and/or” includes any and all combinations of one or more ofthe associated listed items. Expressions such as “at least one of,” whenpreceding a list of elements, modify the entire list of elements and donot modify the individual elements of the list.

As used herein, the terms “substantially,” “about,” and similar termsare used as terms of approximation and not as terms of degree, and areintended to account for the inherent deviations in measured orcalculated values that would be recognized by those of ordinary skill inthe art. Further, the use of “may” when describing embodiments of thepresent invention refers to “one or more embodiments of the presentinvention.” As used herein, the terms “use,” “using,” and “used” may beconsidered synonymous with the terms “utilize,” “utilizing,” and“utilized,” respectively. Also, the term “exemplary” is intended torefer to an example or illustration.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which the present invention belongs. Itwill be further understood that terms, such as those defined in commonlyused dictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art and/orthe present specification, and should not be interpreted in an idealizedor overly formal sense, unless expressly so defined herein.

Embodiments described herein are exemplary only. One skilled in the artmay recognize various alternative embodiments from those specificallydisclosed. Those alternative embodiments are also intended to be withinthe scope of this disclosure. As such, the embodiments are limited onlyby the following claims and their equivalents.

The invention claimed is:
 1. An electronic device, comprising: anintegrated circuit (IC) configured to regulate an output voltage forpowering a light emitting diode (LED); a first transistor configured tobe switched on or off by the IC to inductively couple or decouple a mainpower supply bus voltage from a primary winding of a transformer to asecondary winding of the transformer connectable to the LED; and asecond transistor coupled between the IC and the main power supply busvoltage, and configured to be switched on or off by the IC toselectively provide an IC power supply input voltage to the IC, whereinthe second transistor is configured to be switched on in response to:the IC decreasing the output voltage to less than about one-third of anominal operating voltage of the LED, and detecting that the IC powersupply input voltage is less than a first threshold voltage.
 2. Thedevice of claim 1, wherein the IC is configured to enter a low powermode in response to the IC entering light-off state.
 3. The device ofclaim 1, wherein the second transistor is configured to be turned off inresponse to the IC detecting that the IC power supply input voltage isgreater than a second threshold voltage.
 4. The device of claim 3,wherein the first threshold voltage is greater than anundervoltage-lockout (UVLO) voltage of the LED, and less than the secondthreshold voltage.
 5. The device of claim 3, wherein the IC comprises afirst comparator configured to compare the IC power supply input voltagewith the first threshold voltage, and a second comparator configured tocompare the IC power supply input voltage with the second thresholdvoltage, and wherein the detecting the IC power supply input voltage isbased on the compared IC power supply input voltage.
 6. The device ofclaim 1, wherein the output voltage is decreased by reducing a frequencyof a pulse-width modulated (PWM) signal provided to the IC.
 7. Thedevice of claim 6, wherein the IC is in a light-off state when a dutyratio of the PWM signal is less than 1%.
 8. The device of claim 1,further comprising a flyback power converter comprising the transformerand the IC coupled to a bridge rectifier.
 9. The device of claim 1,wherein the IC is in a light-off state when the output voltage is lessthan about one-third of the nominal operating voltage.
 10. A method forregulating a voltage, comprising: reducing an output voltage coupledwith a light emitting diode (LED) to about one-third of a nominaloperating voltage of the LED with a dimming signal to a voltageregulating integrated circuit (IC); and switching on a transistorcoupled between the IC and a main power supply bus to apply a main powersupply bus voltage to the IC in response to: the IC decreasing theoutput voltage to less than about one-third of a nominal operatingvoltage of the LED, and detecting that a power supply input voltage ofthe IC is less than a first threshold voltage.
 11. The method of claim10, further comprising placing the IC in a low power mode in response tothe IC entering a light-off state.
 12. The method of claim 10, furthercomprising switching off the transistor in response to the IC detectingthat the IC power supply input voltage is greater than a secondthreshold voltage.
 13. The method of claim 12, wherein the firstthreshold voltage is greater than an undervoltage-lockout (UVLO) voltageof the LED, and less than the second threshold voltage.
 14. The methodof claim 10, wherein the reducing the output voltage with the dimmingsignal comprises reducing a frequency of a pulse-width modulated (PWM)signal provided to the IC.
 15. An electronic device, comprising: anintegrated circuit (IC) configured to regulate an output voltagepowering a light emitting diode (LED); and a transistor configured to beswitched on or off by the IC to inductively couple or decouple a mainpower supply bus voltage from a primary winding of a transformer to asecondary winding of the transformer connectable to the LED, wherein theIC is configured to enter a light-off state in response to receiving asignal comprising a duty ratio less than a predetermined threshold, andwherein in response to the IC entering the light-off state, thetransistor is configured to be: switched on when an IC power supplyinput voltage is less than a first threshold voltage, and switched offwhen the IC power supply input voltage is greater than a secondthreshold voltage.
 16. The device of claim 15, wherein the IC isconfigured to enter a low power mode in response to the IC entering thelight-off state.
 17. The device of claim 15, wherein the first thresholdvoltage is greater than an undervoltage-lockout (UVLO) voltage of theLED, and less than the second threshold voltage.
 18. The device of claim15, wherein the signal comprises a pulse-width modulated (PWM) signal.19. The device of claim 18, wherein the IC comprises: a PWM detectioncircuit configured to determine the duty ratio of the signal; and afirst comparator configured to compare the IC power supply input voltagewith the first threshold voltage, and a second comparator configured tocompare the IC input power supply voltage with the second thresholdvoltage.
 20. The device of claim 15, wherein the predetermined thresholdcomprises 1%.
 21. A method for regulating a voltage, comprising:reducing an output voltage coupled with a light emitting diode (LED) toabout one-third of a nominal operating voltage of the LED with a dimmingsignal, to place a voltage regulating integrated circuit (IC) in a lowpower mode; and switching on a transistor configured to inductivelycouple or decouple a main power supply bus voltage from a primarywinding of a transformer to a secondary winding of the transformerconnectable to the LED, the switching on being in response to: the ICentering the low power mode, and an IC power supply input voltage beingless than a first threshold voltage.
 22. The method of claim 21, furthercomprising switching off the transistor in response to the IC powersupply input voltage being greater than a second threshold voltage. 23.The method of claim 22, wherein the first threshold voltage is greaterthan an undervoltage-lockout (UVLO) voltage of the LED, and less thanthe second threshold voltage.
 24. The method of claim 21, wherein thedimming signal comprises a pulse-width modulated (PWM) signal.